#!/usr/bin/env python
#-*- coding: utf-8 -*-

L1 = ['Hello', 'World', 18, 'Apple', None]
L2 = []

for s in L1:
    if isinstance(s, str):
        L2.append(s.lower())
    else:
        L2.append(s)

[s.lower() if isinstance(s, str) else s for s in L1]
